DISEASES OF THE SMALL AIRWAYS

The term 'bronchiolitis obliterans' has been used differently by clinicians and pathologists and encompasses two distinct morphologic entities: classic bronchiolitis obliterans with intraluminal polyps and constrictive bronchiolitis. Bronchiolitis obliterans with organizing pneumonia (BOOP pattern) is characterized by classic bronchiolitis obliterans, with intraluminal polyps affecting mainly respiratory bronchioles and alveolar ducts. This represents a nonspecific reaction to acute lung injury and can be seen in a variety of circumstances. Many patients have an idiopathic syndrome (idiopathic BOOP) with well-defined clinical and radiographic features that characterize it clinically as a form of interstitial (infiltrative) lung disease. Idiopathic BOOP is usually steroid- responsive and has a good prognosis. Constrictive bronchiolitis is characterized by bronchiolar inflammation, peribronchiolar and submucosal scarring, and progressive bronchiolar luminal narrowing. Constrictive bronchiolitis is also seen in a variety of clinical contexts, and it has been described in certain allograft recipients in association with some collagen vascular diseases after certain pulmonary infections and after a number of inhalational injuries. Idiopathic constrictive bronchiolitis is rare and remains an ill-defined group that overlaps with other types of inflammatory bronchiolar lesions. Regardless of the clinical situation in which it occurs, however, constrictive bronchiolitis is usually associated with irreversible airflow obstruction and has a poor prognosis.